Field of the Invention
The invention relates to fill level measuring device with a membrane and a drive unit actuating this membrane, whereby oscillating rods are molded onto the membrane at regular intervals.
Background of the Invention
Fill level measuring devices are known, for example, from German Patent DE 100 23 302 C2. A stack of piezoelectric elements with a tension bolt for actuating the piezoelectric elements serves as the drive unit. The stack of piezoelectric elements is disposed between a preload element, in particular a preload nut, and a pressure ring in contact with the membrane. Even though these fill level measuring devices are perfectly suitable to determine fill levels, one drawback of this type of drive lies in that the preloaded drive stack of piezoelectric elements is released at positive process pressure (excess pressure) due to the distending membrane and the off-center pressure ring. As a result, the mechanical coupling of the piezoelectric elements decreases as the process pressure increases. Such low mechanical coupling leads to decreasing electric amplitudes, which can even lead to complete failure of the drive unit. This effect can be offset by applying a higher bias voltage to the piezoelectric elements, but this is possible only to a limited extent, because the individual piezoelectric elements have a mechanical load limit, resulting in an increased risk of breaking if the bias voltage is too high, and also, if the pressure is too great, depolarization occurs. A further problem is that a higher bias voltage of the drive unit causes structural mechanical stiffening of the membrane.
In order to avoid this disadvantage, i.e. to avoid a release of the stack under positive process pressure, vibration limit switches with a bolted, piezoelectric drive are now also being sold by the applicant, whereby the drive is braced on an intermediate bottom attached to the pipe wall of the housing of the fill level measuring device. Such fill level measuring devices are disclosed, for example, in DE 10 2007 041 349 A1. The bolt in this fill level measuring device no longer functions as a tension bolt, but rather as a pressure bolt. On its end facing the membrane, this pressure bolt is fixedly connected to the aforementioned intermediate bottom, and at its upper end it is provided with a preload element, in particular a preload nut, via which the stack of piezoelectric elements can be pushed toward the center of the membrane, against a pressure piece reaching through the intermediate bottom. Such an arrangement of the fill level measuring device has the advantage that the entire drive unit is loaded with positive process pressure. The electrical amplitude therefore does not decrease with increasing pressure, as is the case in the fill level measuring devices mentioned above.
In this second type of drive, however, it is disadvantageous that the stack of piezoelectric elements is now released at negative process pressure (vacuum). Since the drive side of the membrane is usually under atmospheric pressure, the amount of negative process pressure is physically limited to −1 bar. Assuming thin membranes, however, which would be absolutely necessary, for example, when reducing the size of the fill level measuring device, even this pressure can already lead to a release of the drive. This release of the drive can even occur in applications with higher temperatures, because the axial extension of the bolt here causes additional release.
The objective of the invention is to specify a fill level measuring device that effectively avoids the disadvantages associated with the known fill level measuring devices. The sought after fill level measuring device should be configured in such a way that a sufficiently high electrical signal amplitude is supplied at both positive and negative process pressures, and that the entire assembly thus remains functional.
This task is solved by means of a fill level measuring device described herein.